The demand for wireless communication services, such as cellular mobile telephone (CMT), digital cellular network (DCN), personal communication services (PCS) and the like, requires the operators of such systems to make maximum effective use of the available radio frequency bandwidth. Consider that the system operator must serve an ever increasing number of users in a given geographic territory, while having been allocated only a certain amount of radio spectrum that affords the ability to transmit and receive on only a limited number of radio channels.
In a conventional cellular system, links between the mobile stations and base stations are created using narrowband radio channels. In an effort to make the best use of the allocated radio spectrum, the geographic territory is then divided into a number of sub-areas, called cells. The radio channels are then allocated to the cells such that the amount of interference is minimal and such that capacity is maximized. This is typically accomplished by reusing the same channels within multiple cells located a minimum distance from one another. This distance, called the reuse distance, is determined such that the interference between adjacent base stations is minimal.
Even with this extensive frequency planning, however, service providers are finding that they cannot keep up with the demand for new cellular services, which in some areas has experienced annual growth rates of fifty percent (50%) or more. As such, numerous techniques continue to be proposed to increase cellular mobile telephone system capacity.
One such highly efficient technique was described in a co-pending U.S. patent application Ser. No. 08/331,455 filed by John Doner on Oct. 31, 1994, now U.S. Pat No. 5,649,292 entitled "A Method for Obtaining Times One Frequency Reuse in Communication Systems" and assigned to AirNet Communications Corp., who is the assignee of this application. According to that arrangement, the cells are each split into six radial sectors and frequencies are assigned to the sectors in such a manner as to provide the ability to reuse each available frequency in every third cell. Although this so-called "N=3" reuse scheme is highly efficient, it is not always cost effective. Specifically, it requires at least two complete sets of multichannel transceiver equipment either in the form of sets of multiple individual transceivers or as a broadband transceiver system (BTS) to be located in each cell. Because such multichannel transceiver equipment may end up costing several hundred thousand dollars to deploy, when such a system first comes on line or at other places in the system where the demand is relatively low, it may not be possible to justify the cost of deploying such complex equipment.
Because only a few cells at high expected demand locations will originally require the build out of sectorized cells and on broadband transceiver equipment, one might think that it would be relatively easy to deploy such cells of immediate high demand, and then upgrade the equipment as cell traffic increases. However, this is not as simple as it might seem at first glance. First, low density reuse patterns are typically implemented using a reuse factor of seven, and thus the reuse patterns do not fit well into a reuse grid of three because seven is not divisible by three. Even if the low density reuse pattern is selected to be a multiple of three, such as six, nine or twelve, mixing the N=3 cell patterns with N=6 or N=12 patterns creates unacceptable interference between homologous cells at the periphery of the cell patterns of a particular reuse factor, especially where one attempts to locate a sectorized cell adjacent to an un-sectorized cell.